I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing channel estimation in an orthogonal frequency division multiplexing (OFDM) communication system.
II. Background
OFDM is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (N) orthogonal subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data.
In a wireless communication system, a radio frequency (RF) modulated signal may travel via a number of signal paths from a transmitter to a receiver. If the signal paths have different delays, then the received signal at the receiver would include multiple instances of the transmitted signal with different gains and delays. This time dispersion in the wireless channel causes frequency selective fading, which is characterized by a frequency response that varies across the system bandwidth. For an OFDM system, the N subbands may thus experience different effective channels and may consequently be associated with different complex channel gains.
An accurate estimate of the wireless channel between the transmitter and the receiver is normally needed in order to effectively receive data on the available subbands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of modulation symbols that are known a priori by the receiver, the channel response can be estimated as the ratio of the received pilot symbol over the transmitted pilot symbol for each subband used for pilot transmission.
Pilot transmission represents overhead in the OFDM system. Thus, it is desirable to minimize pilot transmission to the extent possible. This can be achieved by sending pilot symbols on a subset of the N total subbands and using these pilot symbols to derive channel estimates for all subbands of interest. As described below, the computation to derive the channel estimates can be great for certain systems such as, for example, (1) a spectrally shaped system that does not transmit data/pilot near the band edges and (2) a system that cannot transmit data/pilot on certain subbands (e.g., zero or DC subband). There is therefore a need in the art for techniques to efficiently estimate the channel response for these systems.